1. Field of the Invention
This invention relates to a non-magnetic stainless steel which is superior in strength and corrosion resistance and utilized for micro-shafts of video tape recorders (VTR) and electromagnetic valves, and also to a method for manufacturing the stainless steel.
2. Description of the Prior Art
As VTR equipment has become smaller, the driving mechanisms have changed. Most earlier VTR's employed a belt, whereas recent VTR's use a revolving shaft to drive the cartridge directly.
The raw material for a micro-shaft must possess a hardness of more than Hv 500, and its magnetic permeability must not exceed 1.01 after drawing. Conventional SUS420J2 is inferior in non-magnetic properties and cannot meet these requirements.
18Mn-5Cr steel possesses a hardness of over Hv 500 and a magnetic permeability (.mu.) of less than 1.01. This steel, however, is very inferior in corrosion resistance, though superior in hardness and non-magnetic properties after drawing.
Further, low Ni-high Mn stainless steels which possess high strength and non-magnetic properties are, for example: EQU 0.1C-0.6Si-12.5Mn-1.6Ni-17.5Cr-0.35N (ASTM XM-28) EQU 0.05C-0.6Si-13Mn-3.2Ni-17.5Cr-0.32N (ASTM XM-29) EQU 0.1C-0.6Si-16Mn-0.1Ni-18Cr-0.4N (ASTM XM-31) EQU 0.18C-0.6Si-15Mn-1.25Ni-17Cr-0.35N (205)
ASTM XM-28 possesses a hardness exceeding Hv 500, but its magnetic permeability is 1.05.
ASTM XM-29 is superior in that its magnetic permeability is less than 1.01 but it does not possess a hardness of more than Hv 500.
Each of ASTM XM-31 and 205, like XM-29, has a magnetic permeability less than 1.01, but they cannot stably provide a hardness of more than Hv 500. ASTM XM-31 and 205 are inferior also in hot-workability and ductility after drawing because of a large amount of Mn contained therein. Wire drawing of more than 50% was tested and found to be difficult with these steels.
All of these steels are low Ni and high Mn steels, and therefore they are inferior to SUS304 in corrosion resistance and ductility after drawing.